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Expected number of moves to reach the end of a board | Matrix Exponentiation

  • Last Updated : 09 Jun, 2021

Given a linear board of length N numbered from 1 to N, the task is to find the expected number of moves required to reach the Nth cell of the board, if we start at cell numbered 1 and at each step we roll a cubical dice to decide the next move. Also, we cannot go outside the bounds of the board. Note that the expected number of moves can be fractional.

Examples:  

Input: N = 8 
Output:
p1 = (1 / 6) | 1-step -> 6 moves expected to reach the end 
p2 = (1 / 6) | 2-steps -> 6 moves expected to reach the end 
p3 = (1 / 6) | 3-steps -> 6 moves expected to reach the end 
p4 = (1 / 6) | 4-steps -> 6 moves expected to reach the end 
p5 = (1 / 6) | 5-steps -> 6 moves expected to reach the end 
p6 = (1 / 6) | 6-steps -> 6 moves expected to reach the end 
If we are 7 steps away, then we can end up at 1, 2, 3, 4, 5, 6 steps 
away with equal probability i.e. (1 / 6). 
Look at the above simulation to understand better. 
dp[N – 1] = dp[7] 
= 1 + (dp[1] + dp[2] + dp[3] + dp[4] + dp[5] + dp[6]) / 6 
= 1 + 6 = 7

Input: N = 10 
Output: 7.36111 
 

Approach: An approach based on dynamic programming has already been discussed in an earlier post. In this article, a more optimized method to solve this problem will be discussed. The idea is using a technique called Matrix-Exponentiation
Let’s define An as the expected number of moves to reach the end of a board of length N + 1



The recurrence relation will be: 

An = 1 + (An-1 + An-2 + An-3 + An-4 + An-5 + An-6) / 6 
 

Now, the recurrence relation needs to be converted in a suitable format.  

An = 1 + (An-1 + An-2 + An-3 + An-4 + An-5 + An-6) / 6 (equation 1) 
An-1 = 1 + (An-2 + An-3 + An-4 + An-5 + An-6 + An-7) / 6 (equation 2) 
Substracting 1 with 2, we get An = 7 * (An-1) / 6 – (An-7) / 6 
 

Matrix exponentiation technique can be applied here on the above recurrence relation. 
Base will be {6, 6, 6, 6, 6, 6, 0} corresponding to {A6, A5, A4…, A0} 
Multiplier will be 


{7/6, 1, 0, 0, 0, 0, 0}, 
{0, 0, 1, 0, 0, 0, 0}, 
{0, 0, 0, 1, 0, 0, 0}, 
{0, 0, 0, 0, 1, 0, 0}, 
{0, 0, 0, 0, 0, 1, 0}, 
{0, 0, 0, 0, 0, 0, 1}, 
{-1/6, 0, 0, 0, 0, 0, 0} 

 

To find the value:  

  • Find mul(N – 7)
  • Find base * mul(N – 7).
  • The first value of the 1 * 7 matrix will be the required answer.

Below is the implementation of the above approach:  

C++




// C++ implementation of the approach
#include <bits/stdc++.h>
#define maxSize 50
using namespace std;
 
// Function to multiply two 7 * 7 matrix
vector<vector<double> > matrix_product(
    vector<vector<double> > a,
    vector<vector<double> > b)
{
    vector<vector<double> > c(7);
    for (int i = 0; i < 7; i++)
        c[i].resize(7, 0);
 
    for (int i = 0; i < 7; i++)
        for (int j = 0; j < 7; j++)
            for (int k = 0; k < 7; k++)
                c[i][j] += a[i][k] * b[k][j];
    return c;
}
 
// Function to perform matrix exponentiation
vector<vector<double> > mul_expo(vector<vector<double> > mul, int p)
{
 
    // 7 * 7 identity matrix
    vector<vector<double> > s = { { 1, 0, 0, 0, 0, 0, 0 },
                                  { 0, 1, 0, 0, 0, 0, 0 },
                                  { 0, 0, 1, 0, 0, 0, 0 },
                                  { 0, 0, 0, 1, 0, 0, 0 },
                                  { 0, 0, 0, 0, 1, 0, 0 },
                                  { 0, 0, 0, 0, 0, 1, 0 },
                                  { 0, 0, 0, 0, 0, 0, 1 } };
 
    // Loop to find the power
    while (p != 1) {
        if (p % 2 == 1)
            s = matrix_product(s, mul);
        mul = matrix_product(mul, mul);
        p /= 2;
    }
 
    return matrix_product(mul, s);
}
 
// Function to return the required count
double expectedSteps(int x)
{
 
    // Base cases
    if (x == 0)
        return 0;
    if (x <= 6)
        return 6;
 
    // Multiplier matrix
    vector<vector<double> > mul = { { (double)7 / 6, 1, 0, 0, 0, 0, 0 },
                                    { 0, 0, 1, 0, 0, 0, 0 },
                                    { 0, 0, 0, 1, 0, 0, 0 },
                                    { 0, 0, 0, 0, 1, 0, 0 },
                                    { 0, 0, 0, 0, 0, 1, 0 },
                                    { 0, 0, 0, 0, 0, 0, 1 },
                                    { (double)-1 / 6, 0, 0, 0, 0, 0, 0 } };
 
    // Finding the required multiplier
    // i.e mul^(X-6)
    mul = mul_expo(mul, x - 6);
 
    // Final answer
    return (mul[0][0]
            + mul[1][0]
            + mul[2][0]
            + mul[3][0]
            + mul[4][0]
            + mul[5][0])
           * 6;
}
 
// Driver code
int main()
{
    int n = 10;
 
    cout << expectedSteps(n - 1);
 
    return 0;
}

Java




// Java implementation of the approach
class GFG
{
static final int maxSize = 50;
 
// Function to multiply two 7 * 7 matrix
static double [][] matrix_product(double [][] a,
                                  double [][] b)
{
    double [][] c = new double[7][7];
 
    for (int i = 0; i < 7; i++)
        for (int j = 0; j < 7; j++)
            for (int k = 0; k < 7; k++)
                c[i][j] += a[i][k] * b[k][j];
    return c;
}
 
// Function to perform matrix exponentiation
static double [][] mul_expo(double [][] mul, int p)
{
 
    // 7 * 7 identity matrix
    double [][] s = {{ 1, 0, 0, 0, 0, 0, 0 },
                     { 0, 1, 0, 0, 0, 0, 0 },
                     { 0, 0, 1, 0, 0, 0, 0 },
                     { 0, 0, 0, 1, 0, 0, 0 },
                     { 0, 0, 0, 0, 1, 0, 0 },
                     { 0, 0, 0, 0, 0, 1, 0 },
                     { 0, 0, 0, 0, 0, 0, 1 }};
 
    // Loop to find the power
    while (p != 1)
    {
        if (p % 2 == 1)
            s = matrix_product(s, mul);
        mul = matrix_product(mul, mul);
        p /= 2;
    }
    return matrix_product(mul, s);
}
 
// Function to return the required count
static double expectedSteps(int x)
{
 
    // Base cases
    if (x == 0)
        return 0;
    if (x <= 6)
        return 6;
 
    // Multiplier matrix
    double [][]mul = { { (double)7 / 6, 1, 0, 0, 0, 0, 0 },
                                   { 0, 0, 1, 0, 0, 0, 0 },
                                   { 0, 0, 0, 1, 0, 0, 0 },
                                   { 0, 0, 0, 0, 1, 0, 0 },
                                   { 0, 0, 0, 0, 0, 1, 0 },
                                   { 0, 0, 0, 0, 0, 0, 1 },
                                   {(double) - 1 / 6, 0, 0,
                                                0, 0, 0, 0 }};
 
    // Finding the required multiplier
    // i.e mul^(X-6)
    mul = mul_expo(mul, x - 6);
 
    // Final answer
    return (mul[0][0] + mul[1][0] + mul[2][0] +
            mul[3][0] + mul[4][0] + mul[5][0]) * 6;
}
 
// Driver code
public static void main(String[] args)
{
    int n = 10;
 
    System.out.printf("%.5f",expectedSteps(n - 1));
}
}
 
// This code is contributed by PrinciRaj1992

Python3




# Python3 implementation of the approach
import numpy as np
 
maxSize = 50
 
# Function to multiply two 7 * 7 matrix
def matrix_product(a, b) :
    c = np.zeros((7, 7));
     
    for i in range(7) :
        for j in range(7) :
            for k in range(7) :
                c[i][j] += a[i][k] * b[k][j];
    return c
 
# Function to perform matrix exponentiation
def mul_expo(mul, p) :
 
 
    # 7 * 7 identity matrix
    s = [ [ 1, 0, 0, 0, 0, 0, 0 ],
          [ 0, 1, 0, 0, 0, 0, 0 ],
          [ 0, 0, 1, 0, 0, 0, 0 ],
          [ 0, 0, 0, 1, 0, 0, 0 ],
          [ 0, 0, 0, 0, 1, 0, 0 ],
          [ 0, 0, 0, 0, 0, 1, 0 ],
          [ 0, 0, 0, 0, 0, 0, 1 ] ];
 
    # Loop to find the power
    while (p != 1) :
        if (p % 2 == 1) :
            s = matrix_product(s, mul);
             
        mul = matrix_product(mul, mul);
        p //= 2;
 
    return matrix_product(mul, s);
 
# Function to return the required count
def expectedSteps(x) :
 
    # Base cases
    if (x == 0) :
        return 0;
         
    if (x <= 6) :
        return 6;
 
    # Multiplier matrix
    mul = [ [ 7 / 6, 1, 0, 0, 0, 0, 0 ],
            [ 0, 0, 1, 0, 0, 0, 0 ],
            [ 0, 0, 0, 1, 0, 0, 0 ],
            [ 0, 0, 0, 0, 1, 0, 0 ],
            [ 0, 0, 0, 0, 0, 1, 0 ],
            [ 0, 0, 0, 0, 0, 0, 1 ],
            [ -1 / 6, 0, 0, 0, 0, 0, 0 ] ];
 
    # Finding the required multiplier
    # i.e mul^(X-6)
    mul = mul_expo(mul, x - 6);
 
    # Final answer
    return (mul[0][0] + mul[1][0] + mul[2][0] +
            mul[3][0] + mul[4][0] + mul[5][0]) * 6;
 
# Driver code
if __name__ == "__main__" :
 
    n = 10;
 
    print(expectedSteps(n - 1));
     
# This code is contributed by AnkitRai01

C#




// C# implementation of the approach
using System;
 
class GFG
{
static readonly int maxSize = 50;
 
// Function to multiply two 7 * 7 matrix
static double [,] matrix_product(double [,] a,
                                 double [,] b)
{
    double [,] c = new double[7, 7];
 
    for (int i = 0; i < 7; i++)
        for (int j = 0; j < 7; j++)
            for (int k = 0; k < 7; k++)
                c[i, j] += a[i, k] * b[k, j];
    return c;
}
 
// Function to perform matrix exponentiation
static double [,] mul_expo(double [,] mul, int p)
{
 
    // 7 * 7 identity matrix
    double [,] s = {{ 1, 0, 0, 0, 0, 0, 0 },
                    { 0, 1, 0, 0, 0, 0, 0 },
                    { 0, 0, 1, 0, 0, 0, 0 },
                    { 0, 0, 0, 1, 0, 0, 0 },
                    { 0, 0, 0, 0, 1, 0, 0 },
                    { 0, 0, 0, 0, 0, 1, 0 },
                    { 0, 0, 0, 0, 0, 0, 1 }};
 
    // Loop to find the power
    while (p != 1)
    {
        if (p % 2 == 1)
            s = matrix_product(s, mul);
        mul = matrix_product(mul, mul);
        p /= 2;
    }
    return matrix_product(mul, s);
}
 
// Function to return the required count
static double expectedSteps(int x)
{
 
    // Base cases
    if (x == 0)
        return 0;
    if (x <= 6)
        return 6;
 
    // Multiplier matrix
    double [,]mul = {{(double)7 / 6, 1, 0, 0, 0, 0, 0 },
                                { 0, 0, 1, 0, 0, 0, 0 },
                                { 0, 0, 0, 1, 0, 0, 0 },
                                { 0, 0, 0, 0, 1, 0, 0 },
                                { 0, 0, 0, 0, 0, 1, 0 },
                                { 0, 0, 0, 0, 0, 0, 1 },
                                {(double) - 1 / 6, 0, 0,
                                                0, 0, 0, 0 }};
 
    // Finding the required multiplier
    // i.e mul^(X-6)
    mul = mul_expo(mul, x - 6);
 
    // Final answer
    return (mul[0, 0] + mul[1, 0] + mul[2, 0] +
            mul[3, 0] + mul[4, 0] + mul[5, 0]) * 6;
}
 
// Driver code
public static void Main(String[] args)
{
    int n = 10;
 
    Console.Write("{0:f5}", expectedSteps(n - 1));
}
}
 
// This code is contributed by Rajput-Ji

Javascript




<script>
 
// Javascript implementation of the approach
var maxSize = 50;
 
// Function to multiply two 7 * 7 matrix
function matrix_product(a, b)
{
    var c = Array.from(Array(7),
                 () => Array(7).fill(0));
 
    for(var i = 0; i < 7; i++)
        for(var j = 0; j < 7; j++)
            for(var k = 0; k < 7; k++)
                c[i][j] += a[i][k] * b[k][j];
                 
    return c;
}
 
// Function to perform matrix exponentiation
function mul_expo( mul, p)
{
     
    // 7 * 7 identity matrix
    var s = [ [ 1, 0, 0, 0, 0, 0, 0 ],
              [ 0, 1, 0, 0, 0, 0, 0 ],
              [ 0, 0, 1, 0, 0, 0, 0 ],
              [ 0, 0, 0, 1, 0, 0, 0 ],
              [ 0, 0, 0, 0, 1, 0, 0 ],
              [ 0, 0, 0, 0, 0, 1, 0 ],
              [ 0, 0, 0, 0, 0, 0, 1 ] ];
 
    // Loop to find the power
    while (p != 1)
    {
        if (p % 2 == 1)
            s = matrix_product(s, mul);
             
        mul = matrix_product(mul, mul);
        p = parseInt(p / 2);
    }
    return matrix_product(mul, s);
}
 
// Function to return the required count
function expectedSteps(x)
{
     
    // Base cases
    if (x == 0)
        return 0;
    if (x <= 6)
        return 6;
 
    // Multiplier matrix
    var mul = [ [ 7 / 6, 1, 0, 0, 0, 0, 0 ],
                [ 0, 0, 1, 0, 0, 0, 0 ],
                [ 0, 0, 0, 1, 0, 0, 0 ],
                [ 0, 0, 0, 0, 1, 0, 0 ],
                [ 0, 0, 0, 0, 0, 1, 0 ],
                [ 0, 0, 0, 0, 0, 0, 1 ],
                [ -1 / 6, 0, 0, 0, 0, 0, 0 ] ];
 
    // Finding the required multiplier
    // i.e mul^(X-6)
    mul = mul_expo(mul, x - 6);
 
    // Final answer
    return (mul[0][0] + mul[1][0] +
            mul[2][0] + mul[3][0] +
            mul[4][0] + mul[5][0]) * 6;
}
 
// Driver code
var n = 10;
 
document.write(expectedSteps(n - 1));
 
// This code is contributed by rrrtnx
 
</script>
Output: 
7.36111

 

Time Complexity of the above approach will be O(343 * log(N)) or simply O(log(N)).
 

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